1. Field of the Invention
The present invention relates to a method and apparatus for searching line data in a graphic data base system.
2. Description of the Related Art
Recently, computer mapping systems have been extensively used to retrieve graphic data of a desired area, and to display a map of that area based on the graphic data in response to access to a graphic data base system from a terminal. The graphic data base system is built by representing each line or graphic element on a map in the form of line data, and by storing the line data into a file. Here, the line data usually consists of sequentially arranged coordinates of characteristic points on a line.
Such computer mapping systems have already been used in the fields of public facility management systems, road management systems, housing management systems, administrative management systems or the like. Thus, the graphic data base systems have been built with regard to maps representing the state of burial facilities such as gas pipeline networks, water pipes, communication lines, power lines or the like, and with regard to road maps and maps representing the state of allocation of real estate.
FIG. 1 is a block diagram illustrating a general computer mapping system. In this figure, reference numeral 70 designates a host computer; 71, a large capacity disk storage containing a graphic data base; 72, another large capacity disk storage comprising a work area 72b and an area for storing an application program 72a executing the input, searching or the like of the line data; 73, an attribution data input portion; 74, a display portion for confirming the input of the line data; 75, a digitizer for inputting the coordinates of each characteristic point on each line to be inputted; 76, a terminal used for searching and outputting the line data; and 77, a plotter for printing on recording paper a displayed map.
The line data comprises coordinate data and attribution data (that is, line type tag). The coordinate data is inputted by sequentially designating the characteristic points on a line, which is a graphic element, on a map which is set on the digitizer 75. The attribution data is obtained by designating the type of a line segment, such as a solid line, a broken line, straight line or a semicircle, between two adjacent characteristic points by the attribution data input portion 73. The attribution data may be omitted when the entire lines to be inputted are represented by a single type of a line such as a solid straight line.
FIG. 2 illustrates a line data format cited from page 44 of "Digital Mapping", directed by the Geographical Survey Institute of Japan, and published by Kajima Institute Publishing Co., LTD., 1991. The line data format comprises a header 81 followed by a data area 82. Each sector of the data area 82 includes three kinds of data: the x-coordinate and y-coordinate of each characteristic point stored in the first and second portions of the sector, and a line type tag as the attribution data stored in the third portion.
Since the area to be handled by the graphic data base system is very broad such as Tokyo, the area is usually divided into many rectangular unit areas, each of which is represented by a piece of map. For example, the unit area is defined as a rectangular area 2 km from east to west by 1.5 km from north to south, that is, the map of a unit area is 80 cm.times.60 cm on a national large scale map of Japan whose reduced scale is 1/2500.
The line data of such maps are generally stored in the disk storage 71 on the block basis. Each of the blocks contains the line data of each one map. Each map has a map number which is used as a key for searching the map.
FIG. 3 illustrates an example of the map which is accessed from the terminal 76. When the host computer 70 issues a command demanding the output of the map 85 by using a street name or the like as a key, a rectangular output domain 86 is set in the object map 85 (the object map here refers to a map which is retrieved from the file, and whose portion or portions are outputted from the terminal 76). The application program 72a reads the line data group stored in a block of the graphic file associated with the number of the map 85 from the disk storage 71, that is, reads the entire line data of the line data group associated with the object map 85 including those of the output domain 86, and transmits the line data group to the terminal 76. When the terminal 6 receives the line data group, it selects only the line data necessary to output the output domain 86. Incidentally, the term "rectangle" includes a square in this specification.
Thus, even if only a part of the line data of the object map 85 is demanded from the terminal 76, the host computer 70 in the conventional system must transmit the entire line data associated with the object map 85. In other words, line data unnecessary for outputting the output domain 86 is transmitted to the demanding terminal 76 through a communication line, and the terminal 76 selects the line data necessary for outputting the output domain 86. As a result, the response time from the demand to the output of the output domain 86 is lengthened. To overcome this problem, an expensive serial line of 64 kbps must be used instead of an inexpensive serial line of 9.6 kbps, for example. In addition, the terminal must temporarily store the line data unnecessary for outputting the output domain, which requires a large capacity storage in the terminal 76.
For example, let us suppose that an area of 20 cm.times.15 cm is selected from the 80 cm.times.60 cm map of a unit area of 2 km.times.1.5 km on the national large scale map of Japan whose reduced scale is 1/2500 so that the selected area is to be displayed on a screen without reduction. In this case, if the line density is uniform, the line data as much as 16 times the necessary amount for outputting the area of interest (that is, (80 cm.times.60 cm)/(20 cm.times.15 cm)) must be transmitted from the host computer 70 to the terminal 76.